With an increase in transmission capacity, an expansion of repeater spans, and an increase in the number of fibers in a submarine cable communication system in recent years, electric power required in a submarine branching unit is increasing. In order to supply higher power to a submarine branching unit, voltage of power supplied to the submarine branching unit needs to be increased. To that end, a submarine branching unit that is capable of switching a feed circuit at high voltage when switching a communication route due to a failure in a submarine cable, for example, is demanded.
FIG. 11 is a diagram illustrating connections between a submarine branching unit (hereinafter abbreviated as “BU”) and land stations, which are related to the present invention. In FIG. 11, a BU 901 is connected to land stations 991, 992, and 993 through submarine cables that include power feeding lines, and switches a feed circuit in response to a signal received from any of the land stations 991, 992, and 993. By the switching, the feed circuit of the BU 901 can be connected to a sea earth (hereinafter abbreviated as “SE”) under the sea (the right-hand diagram in FIG. 11) or disconnected from the sea earth (the left-hand diagram in FIG. 11).
FIG. 12 is a diagram illustrating an example of the feed circuit of the BU 901. Internal circuitry 910 includes an electrical circuit that processes optical signals to be transmitted. The feed circuit supplies power to the electrical circuit. As illustrated in FIG. 12, the feed circuit of the BU 901 includes three relays RL1 to RL3. By controlling the relays RL1 to RL3, a state of grounding of the feed circuit of the BU 901 to the SE can be switched. The left-hand side of FIG. 12 illustrates a state in which the internal circuitry is disconnected from the sea earth and the right-hand side of FIG. 12 illustrates a state in which a grounding potential of the internal circuitry is dropped to the sea earth.
Further, in relation to the present invention, PTL 1 describes a technique relating to power feed switching of a submarine branching unit.